1. Field of the Invention
The present invention relates to equipment for processing electronic device substrates and products, and more particularly to equipment for drying products after cleaning.
2. Prior Art
Electronic device substrates or products, such as semiconductor wafers for integrated circuits or metallic platters for hard disk memories, are processed through various sequences of steps. Following some steps, the surface of the substrate is cleaned with an appropriate cleaning agent, and then rinsed in deionized water, which should be dried off without leaving streaks, stains, or spots on the product surface.
Products are dried according to one prior art method in spin dryers provided, for example, by the Flurocarbon Company, the F.S.I. Company, or the Semitool Company. Products are loaded coaxially into a rotor, which is supported within a stainless steel bowl. The products are spun at a high RPM within the bowl on an axis which is inclined from the horizontal at an angle of fifteen degrees to prevent adjacent product surfaces from forming a meniscus which could trap water and contaminants. For the axis to be thusly inclined requires that the products be positioned in three dimensions, which makes the product handling process difficult and expensive to automate. Spin dryers typically have a maximum capacity of one boat containing 25 wafers or products, creating a bottleneck in processing lines otherwise processing two boats at a time. The boat spinning mechanism uses moving parts and bearings which can release contaminating particles into the drying chamber and onto the products, or generate electro-static discharges (ESD) which can damage electronic circuit elements. Boats spinning at a high RPM are difficult to balance and may rattle the products, which is particularly undesirable for fragile gallium arsenide wafers or large disks. To spin off a one micron particle requires a tremendous centrifugal force which is difficult to achieve in a spin dryer. Water may become stagnant while standing for long periods of time in dead end piping systems. The bowl surface maybe glass beaded and heated which aids in drying the water droplets, but provides crevices for trapping particulates which may be shed during expansions and contractions caused by heating and cooling cycles in subsequent drying operations.
To mitigate some of the problems of spin dryers mentioned above, products are sometimes dried in vapor degreasers as shown for example by U.S. Pat. No. 3,375,177 and 4,098,005. However, typical vapor degreasers have been intended for use in heavy industrial settings, and have not been satisfactory for use in semiconductor fabrication clean room environments. K & S Systems provides a vapor degreaser-dryer preceded by a rinser tank using mechanical parts which can introduce contaminants into the vapor degreaser/dryer chamber, and followed by a holding tank, which occupies an undesirably large footprint area in a clean room. Conventional electrically heated vapor degreasers have lacked effective thermal controls, sometimes overheating to ignite combustible solvent vapors. To reduce the risk of fires, some vapor degreasers have used Freon(R) as a solvent. However, Freon(R) and some other solvents contain carbon and organic molecules which tend to stick to substrate surfaces being cleaned. The K & S vapor degreaser heats isopropyl alcohol to 82 degrees centigrade using electric block heaters which present a fire hazard. In a degreaser provided by Delta Sonics Inc., a solvent vapor cloud is condensed by cooling coils using tap water at unregulated temperatures, preventing good control over the temperature and elevation of the vapor cloud. Vapor degreasers having an open top can emit solvents into the atmosphere in contravention of current strict U.S. safety codes.
Most objectionably, in prior art vapor degreaser systems in which solvent is allowed to remain, the used solvent soon becomes dirty, loses its effectiveness, contaminates the clean solvent reservoir, and needs to be replaced frequently (within approximately three hours), decreasing the yield, and increasing the expense, of the product processing system. Reaching a four percent content of water in isopropyl alcohol leaves products with objectionable spots which are difficult to remove by repeating the steps of cleaning and drying with a new batch of clean solvent. Finally, as semiconductor device designers continue minimizing feature sizes, it becomes essential to remove smaller size residues from products.
The remains, therefore, a need for a substrate drying system with better control of solvent purity and better control of solvent vapor cloud temperature and elevation, without using moving parts which could rattle delicate products or introduce particle contaminates or electrostatic discharges, and with reduced danger of combustion. There is also a need for a dryer system occupying a reduced footprint for use in a clean room environment.